The introduction of a foreign object into the vasculature of a patient presents obviously complex problems. Specifically, both the object and its delivery system must be capable of being advanced into the vasculature for its intended purpose, without unduly traumatizing the patient. Further, this advancement must be accomplished accurately and with great precision. To this end, interventional systems are typically designed to be as smooth as possible, to have as small a profile as possible, and to be as easily controlled as possible. These design characteristics, however, do not directly address the different set of problems which are confronted when, after being advanced into the vasculature by a delivery system, an object is thereafter deployed or disengaged from the delivery system to remain in situ.
In order to deploy or disengage an object from a delivery system in the vasculature of a patient, it is necessary to somehow reconfigure the object and the delivery system for this purpose. The structures used for these functions, however must not be allowed to interfere with the aforementioned task of advancing the object into the vasculature. Furthermore, the structure used to deploy or disengage an object from its delivery system must not itself create problems which would traumatize the patient while the system is in the vasculature. In sum, it is important for a delivery system to function reliably and safely. It happens that several devices have been disclosed which are directed toward this result.
U.S. Pat. No. 3,902,501 which issued to Citron et al. for an invention entitled "Endocardial Electrode," and which is assigned to the same assignee as the present invention, discloses an interventional device for the deployment of a medical electrode. Specifically, the invention disclosed by Citron et al. incorporates a shroud which is axially fixed in its position on the device. As so positioned, the shroud overlaps and holds the exposed ends of the electrode's tines against the electrode body until the tines are to be deployed. To deploy the tines, a balloon on the electrode body is inflated to withdraw the tines from the shroud. Another example of this same basic deployment scheme, but for a slightly different application, is provided by U.S. Pat. No. 4,950,227 which issued to Savin et al. for an invention entitled "Stent Delivery System." According to the invention of Savin et al., a stent is positioned over the balloon of a balloon catheter. A pair of sleeves are axially fixed on the device with one end of each sleeve anchored to the catheter while the other end overlaps a respective end of the stent to hold the stent on the balloon. The result is that the sleeve is fixed at an axial location on the catheter. In an action similar to that disclosed for the device of Citron et al., when the balloon is inflated, the ends of the stent are withdrawn from their respective sleeve and the stent is thus deployed.
A difficulty not resolved by either the Citron et al. device or the Savin et al. device is the fact that the sleeves of Savin et al., like the shroud of Citron et al., are axially fixed and therefore substantially immobile. Stated differently, both of these devices rely solely on a change in the configuration of the object being deployed to disengage the object from the shroud or sleeves which hold the object against the balloon. Neither of these devices rely on a mechanical change in either the shroud or the stent for this disengagement. Thus, because the shroud (Citron et al.) or the sleeves (Savin et al.) do not mechanically withdraw axially from the tines or stent during deployment, less of the tines or stent can be covered by the respective shroud or sleeves than would otherwise be possible prior to deployment. Consequently, with less overlapping coverage, there is an increased risk of premature dislodgment of the object being deployed, and an increased risk of a system malfunction during deployment.
In light of the above it is an object of the present invention to provide a system for selectively holding a stent on a balloon delivery catheter which incorporates retainers that will mechanically shorten with an inflation of the balloon to facilitate disengagement of the stent from the system. Still another object of the present invention is to provide a system for selectively holding a stent on a balloon delivery catheter which effectively covers the projecting ends of a stent to protect the patient from snags, hang-ups or entanglements that might otherwise occur during advancement of the system into the vasculature of the patient. Yet another object of the present invention is to provide a system for selectively holding a stent on a balloon delivery catheter which includes stent retainers that revert to a low profile after stent deployment to facilitate withdrawal of the system from the vasculature of the patient. Another object of the present invention is to provide a system for selectively holding a stent on a delivery catheter which is relatively easy to manufacture, simple to use and comparatively cost effective.